Animations not working after renaming vertex groups?
Channels

An action consists of several named channels.



Each channel is assigned to a named bone. The channels are grouped by bone name for easier identification. 

When you have a different armature you get different assignements.



If you rename a bone Blender automatically changes the assignment and the group for you.



But it can do that only on known actions. That means it does this on assigned actions only. There can be only one assigned -> so it renames the current one only.

If you assign an action (via ActionEditor) that belongs to an armature with bones of different names, the channels lose the bone assignment:



This action will not play with this armature as there is no bone to play the channel. 

You can rename such unassigned channels. Be aware this removes the previous assignment.

Vertex Groups

You can assign vertices to bones by assigning them a vertex group with the name that matches the bone name.



When the bone gets animated the animation engine considers the vertices that are part of the matching vertex group.

When you rename vertex groups you need to rename the according bones too. Otherwise the animation can't be matched. Blender will not automatically rename the bones. Re-parenting to the armature will (dependent on the options) result in vertex groups that match the bone names. But these are not the vertex groups you just renamed.

Conclusion


When you rename bones, ensure to rename the channels and the vertex
groups too. 
When you rename vertex groups ensure to rename bones and
channels too.


I hope it helps